De-hum and Cassette Tape Restoration

You digitized a family cassette or an old demo tape — and instead of warm tape you hear a constant hum, hiss in the pauses, and the odd click. That is the usual analog-transfer picture. Here is how to tell the problems apart, what de-hum actually does, and how to restore the recording online without installing plugins.

What breaks cassette audio after transfer

Three different defects get mixed up and “fixed” with one EQ sweep:

1. Mains hum — a tone at 50 Hz (EU/CIS) or 60 Hz (US) plus harmonics at 100/150 Hz and up. It sounds like a low “bzz” that never leaves the silence.
2. Tape hiss — broadband noise up top. Louder in pauses; soft passages “sand” a little.
3. Clicks and crackle — short impulses from vinyl, bad contacts, or splices. Spikes, not constant noise.

Separately there is rumble — infrasonic junk from the deck or turntable mechanics. Cut that with a high-pass before de-hum, not by gut-feeling bass cuts.

What de-hum means in plain language

De-hum is targeted removal of mains interference. The algorithm finds a narrow peak at 50/60 Hz (often automatically from the spectrum) and places narrow notch filters on the fundamental and harmonics. Musical bass is wider than those “needles,” so at moderate depth the tone stays largely intact.

What de-hum does not do:

• it does not wipe tape hiss by itself — that needs spectral denoise;
• it does not rebuild clipped peaks — that is the declipper;
• it does not replace loudness/balance mastering — cleanup happens before the final chain.

A product walkthrough of the modules lives on the audio restoration landing page.

Cassette restoration order that actually works

Order matters: narrow defects first, then broadband noise, then impulses, and only then mastering.

1. Transfer with headroom. Capture to WAV/FLAC; do not slam into 0 dBFS. Input clipping will not be fixed by de-hum.
2. Rumble / high-pass. Remove infrasound below useful bass (often around 30–40 Hz) so the limiter is not fed junk.
3. De-hum. Auto 50/60 Hz, strength from light to medium. Listen to pauses and long bass notes.
4. Soft hiss denoise. Goal: quieter pauses, not a dead studio. Aggressive denoise kills air and cymbal attack.
5. De-click. Only if you hear clicks; on a clean cassette, ease it off or disable it.
6. Mastering. Platform LUFS/True Peak comes after the noise floor is even.

In Magic Master that sequence is bundled as the Restore old recording intent: open in the app.

How to check the result by ear and spectrum

• Pauses between phrases. Hum should drop hard; hiss should get quieter without a knife-cut silence.
• Bass and kick. No new holes or one-note woofer throb — that usually means the notch is too wide or too deep.
• Level-matched A/B. The brain hears a quieter file as “cleaner” — compare honestly.
• Spectrum. The thin 50/60 Hz line dips; the broadband top eases a little without a comb across the whole band.

If after cleanup the track is still a dynamics brick, that is over-compression, not cassette noise — see dynamics restoration.

Common mistakes

• Shelving the whole low end −12 dB “against hum” — you lose tape warmth.
• Maxing denoise — plastic vocals and dead cymbals.
• Mastering before de-hum — the limiter stamps hum into loud moments, and the notch then hurts the music more.
• Treating ground-loop hum in a studio mic take like a cassette preset — sometimes you need a different approach.

In short

• De-hum targets narrow 50/60 Hz mains interference and its harmonics.
• Tape hiss and clicks are separate modules (denoise / de-click).
• Order: rumble → de-hum → denoise → de-click → mastering.
• Online path without a DAW: audio restoration and the Restore old recording intent.
• Sibling articles: tape hiss / denoise, vinyl clicks / de-click.

Your first basic master per day is free; PRO restoration modules use the same token model — there is no separate “de-hum fee.”